[Feature] Pad initial target for template building

[alessandrofelder]

Is your feature request related to a problem? Please describe.
The choice of initial target may cause small artefacts during template building. This is because all samples are rigidly registered to it, and larger samples may have parts that fall outside the initial target, which get cropped.

Describe the solution you'd like
We pad the initial target sufficiently.

Describe alternatives you've considered
Choose the largest image as the initial target. This may not be possible because e.g. the largest in AP may be different to the largest in LR extent.